Digital data is stored and transferred for many applications. Communications is becoming more and more digital. For example, telephony, which was originally analog throughout the world is now almost totally digital. Also, musical performances are now digitally recorded and distributed on CDs. Visual material that had previously been distributed via film and/or VHS tape is now contained on DVDs and Blue Ray disks; television broadcasting has now also entered the digital domain.
Analog “front ends” for digital receivers are thus needed to support the various digital technologies. When a CD, DVD or hard drive is accessed, a receiver is needed to boost the relatively weak signal into a stronger representation that may be properly interpreted in the digital domain. In some cases (such as, for example, with a CD or DVD), a laser is focused on a moving track that causes the light that reaches the receiver to vary as a function of time. The receiver includes a type of opto-electronic conversion device (usually, a photodiode) that converts the received optical signal into a relatively weak electrical signal. One task of the receiver is to amplify this weak signal to a level sufficient to discriminate between the transmission of digital “one's” and digital “zero's”.
Receivers are also a necessary component of communication systems. For example, in a digital fiber-optic link, an optical signal is conveyed over considerable distances by an optical fiber. In some configurations, an optical receiver converts this relatively weak optical signal into a (weak) electrical signal via, for example, a photodiode; electrical amplification is then used to increase the level of the electrical signal to a point where it can be reliably converted into a digital stream of one's and zero's.
All of the above examples require digital, baseband receiver circuits to convert a weak electrical signal into a more robust form that can be readily converted into a digital stream of one's and zero's. A baseband receiver, in contrast to a tuned broadcast receiver, operates with signals where the important frequency range extends from nearly DC (zero frequency) to a level comparable with the bit rate used for transmission.
For a given type of input signal (i.e., optical or electrical) and data encoding method, the main figure of merit for a receiver is its sensitivity. Receiver sensitivity in the digital domain is related to the Bit Error Rate (BER) of the system. When a weak digital signal is received, an occasional bit may be incorrectly labeled (namely, defined as a digital “one” when a digital “zero” was originally transmitted, or vice versa). The ratio of mis-labeled bits to the total number of bits received is thus defined as the BER. For example, in the Ethernet Standards (IEEE 802.3), the minimum level of performance is specified as a BER of 10−12. Modern digital data systems now require a BER on the order of 10−6 to 10−12, depending upon the application. Stated differently, the receiver sensitivity is the minimum signal power needed to achieve the specified BER. If the receiver is presented with a signal below its sensitivity limits, it will have a much higher BER, since random fluctuations (generally attributed to “noise sources” in the receiver) will cause the apparent signal to be mis-interpreted upon occasion.
Receiver sensitivity is important inasmuch as greater sensitivity may indicate that a specific receiver will operate under a greater number of adverse conditions (e.g., poor media—in the case of digital data recordings; greater distances—in the case of transmission systems). For example, in the case of optical fiber transmission systems, a 2 dB improvement in receiver sensitivity may lead to an increase of 5-10 km in the transmission distance signal range.
Optimizing receiver sensitivity is not trivial, and receiver circuits are often designed as a compromise between competing elements, such as noise versus bandwidth. A need therefore remains for a receiver configuration that is not constrained by the competing interests of maintaining a low noise level and a relatively wide operating bandwidth.